The present invention relates to conjugated polymers and copolymers for use in optoelectronic devices containing substituent groups that promote charge transport or charge injection. More particularly, the present invention is drawn to conjugated polymers and copolymers with tunable charge injection and transport ability and to optoelectronic devices fabricated with such polymers and copolymers.
Semi-conducting conjugated polymers combine the features of low cost polymer processing with attractive optoelectronic properties. Electroluminescent devices based on poly(p-phenylene vinylene) (PPV) were first described by Burroughes et al. in 1990 [Burroughes, J. H., et al., Nature, vol. 347, pp. 539-41, 1990]. The light emission in this system is based on the formation of singlet exciton as a result of double charge injection into the emissive polymer. Numerous conjugated polymers have been reported to be highly luminescent materials suitable for light-emission applications [Kraft, A., et al., Angew. Chem. Int. Ed, vol. 37(4), pp. 402-28, 1998]. With appropriate device engineering, PPV based conjugated polymers can also be employed as the active material to produce photovoltaic current under light irradiation. [Granstrom, M., et al., xe2x80x9cLaminated Fabrication of Polymeric Photovoltaic Diodes,xe2x80x9d Nature, vol. 395 (6699), pp. 257-60, 1998].
An electroluminescent or light-emitting device (LED) is usually obtained by sandwiching a conjugated polymer thin film between two electrodes. In order to see the light emission, at least one of the electrodes should be transparent, and in most cases indium-tin oxide (ITO) coated on either a glass substrate or a plastic substrate is used. ITO is normally used as the anode due to its high work function. A low work function metal, such as magnesium, calcium, or aluminum, is usually used as the cathode metal electrode. Under a forward bias (anode wired to positive and cathode wired to negative), electrons are injected into the lowest unoccupied molecular orbital (LUMO, or the lowest position of the conduction band), and holes are injected into the highest occupied molecular orbital (HOMO, or the highest position of the valence band).
As a result of charge transport, some of the electrons and holes may recombine to form an excited state (called siglet exciton) that is annihilated to produce light emission corresponding to the band gap of the conjugated polymer. When the electrodes and device configuration are fixed, the light emission and emission efficiency of the polymer LED is dependent on the nature of the conjugated polymer.
For most conjugated polymers, hole injection (or p-doping) is more favorable than electron injection (n-doping). The unbalanced charge injection and transporting ability of these conjugated semi-conducting polymers result in low efficiency of polymer LEDs, that is, low conversion of electrons to emitted photons. To enhance electron injection for polymer LEDs, one common method is to use a low work function metal as the anode, such as calcium. One drawback of using calcium is that it is extremely sensitive to air.
One approach to facilitate charge injection and transport is to design double layer polymer LEDs. Such devices can include a charge-transporting layer to facilitate electron injection, coupled with a luminescent polymer layer. The use of an appropriate charge-transporting layer can provide a closer match of the cathode to the LUMO (for electron injection) or a closer match of the anode to the HOMO (for hole injection) to facilitate easy charge injection (electrons or holes) into the active luminescent material. For instance, in a device of ITO/polymer/electron-transporter/A1, the electron-transporting layer can, on the one hand, enhance electron-injection and transporting ability, and on the other hand, block hole penetration to the A1 cathode.
Many researchers have been developing new luminescent polymers with enhanced electron affinity. Adding strong electron affinity groups, e.g., cyano, onto a PPV backbone exemplifies efforts to lower the LUMO of a polymer and enhance the electron injection ability. With enhanced electron injection of luminescent polymers, air stable metals, such as aluminum, can be used without loss of electroluminescent efficiency. [N. C. Greenham et al., Nature, vol. 365, pp. 628-30, 1993.] Other luminescent polymers containing electron deficient heterocycles, like oxadiazoles, oxathiazole, pyridine, etc., have been exemplified as electron transporting and hole blocking materials. [X.-C Li, et al., xe2x80x9cSynthesis and Properties of Novel High Electron Affinity Polymers for Electroluminescent Devices,xe2x80x9d ACS Symposium Series, vol. 672, pp. 322-44, 1997.] Due to xe2x80x9cover tuningxe2x80x9d of the electron affinity in these high electron affinity conjugated polymers, hole transporting materials must be used to achieve high efficiency electroluminescence.
To improve the performance of luminescent conjugated polymers with balanced charge injection transporting ability, some researchers have used polycondensation polymerization methods to obtain conjugated polymers containing bipolar pairs of oxadiazoles/triamine [J. Kido, et al., Chem. Lett., p. 161, 1996], oxadiazoles/carbazole [Z. Peng, et al., Chem. Mater., vol. 10, pp. 2086-90, 1998], oxadiazoles/thiophene [W. L. Yu, et al., Macromolecules, vol. 31, pp. 4838-44, 1998], and cyano/triaryl amine [X.-C. Li, et al., Chem Mater., vol 11, pp. 1568-75, 1999]. The general principle of this method can be described in Equation 1, below:
nM1+nM2xe2x86x92(M1M2)nxe2x80x83xe2x80x83(1)
The success of this method (M1M2)n depended on the selection of a suitable pair of bipolar moieties that provided the desired balance of charge injection/transport ability. Furthermore, polycondensation reactions between two different monomer moieties are not easily or economically used to obtain luminescent polymers with controlled charge transporting ability. Copolymers have been considered as an alternative approach to modify the final polymer properties, such as mechanical strength, and to provide a good balance between rigid strength and flexible toughness of a polymer. However, because the monomers used have been principally vinyls, the resulting polymers are not conjugated polymers. [X.-C. Li, et al., Adv. Mater., vol. 11, p. 898, 1995.]
It will be appreciated that there is a need in the art for conjugated polymers and copolymers that can be synthesized with tailored charge injection and transport ability. It will be further appreciated that there is a need for such conjugated polymers and copolymers in the fabrication of optoelectronic devices.
The present invention is directed to conjugated polymers and copolymers combining strong luminescent properties and balanced charge transporting/injection properties. The present invention also includes methods of manufacturing such polymers and copolymers, and to optoelectronic devices fabricated with such polymers and copolymers.
Contrary to the reaction of equation 1, which requires a 1:1 ratio of monomers M1 and M2, the present invention provides a conjugated luminescent polymer with tunable charge transport prepared according to the following polymerization reaction:
mnM1+nM2xe2x86x92(M1)m(M2)nxe2x80x83xe2x80x83(2)
wherein M1 is a monomer having at least two reactive functional groups and at least one chemically bonded charge transporting chromophore group possessing electron-withdrawing character and M2 is a monomer having at least two reactive functional groups and at least one chemically bonded charge transporting chromophore group possessing electron-donating character, wherein m and n are stoichiometric quantities of the monomers M1 and M2, respectively, wherein m and n are varied to tune the charge transport property of the conjugated luminescent polymer.
The monomers preferably are aromatic compounds or hetero-aromatic compounds with at least two reactive functional groups. The functional groups are selected to be self-polymerizable and/or co-polymerizable with another co-monomer under certain chemical and physical conditions. The monomers preferably include aromatic or hetero-aromatic ring(s), like aryl, substituted aryl, benzene, substituted benzene, naphthalene, substituted naphthalene, fluorene, substituted fluorene, thiophene, substituted thiophene, pyridine, substituted pyridine, quinoline, substituted quinoline, oxadiazole, triazole, thiazole, benzothiazole, benzothiophene, and/or multiple carbon double bonds such as vinyl, substituted vinyl, acetyne, etc.
By varying the ratio of different monomers (M1, M2, M3, etc.), the total balance between electron and hole transport can be readily tuned as desired. Electron-withdrawing and/or electron-rich groups or chromophores are chemically linked to the conjugated polymers/copolymers as side functional groups. Statistic copolymers of conjugated polymer segments with electron withdrawing and electron-rich side chromophores provide easy fine-tuning of charge transporting/injection ability for the luminescent polymers.
Typical reactive functional groups include, but are not limited to, halide, aldehyde, nitrile methyl, halide methyl, sulfonium methyl, boronic acid, boronic ester, amino, hydroxide, thiol, ethylene, acetyne, trimethyl silane, trimethyl tin, lithium, Grignard group, and chlorosilane. Examples of some currently preferred functional groups include chloromethyl, bromomethyl, and sulfonium methyl which allow 1,6-polymerization by the formation of p-xylylenes to form a conjugated polymer of poly(a rylene vinylene). The reactive functional groups are preferably the same or chemically similar on each monomer to allow polymerization and/or copolymerization reaction between monomers according to the stoichiometric quantity of each monomer. As used herein, chemically similar functional groups mean that the functional groups have the same or analogous chemical reactivity under the equivalent chemical and physical conditions. Similar functional groups also include functional groups that may undergo a chemical change to form the same or very similar reactive intermediates or follow the same chemical reaction mechanism. One example of chemically similar functional groups includes halide substituent groups, such as chloro- and bromo- or other known leaving groups.
The monomer reactants may be chemically linked with one or more functional substituents that enhance either electron transporting or hole transporting. The monomer reactants may also include a solubilizing functional group such as alkyl, alkoxy, silane, aryl, or heteroaryl.
Some typical electron-withdrawing charge transporting chromophore groups that may be used in accordance with the present invention include, but are not limited to, aromatic oxadiazoles, heteroaromatic rings, cyano groups, and mixtures thereof combined with phenyl or vinyl double bonds. Examples of some currently preferred heteroaromatic rings include pyridine, quinoline, oxadiazole and quinoxaline.
Some typical electron-donating charge transporting chromophore groups that may be used in accordance with the present invention include, but are not limited to, benzene, aromatic amines, carbazoles, thiophenes, farans, and mixtures thereof combined with phenyl or vinyl double bonds.
Additional monomer reactants (M3, M4, M5, etc.) can be used in the polymerization reaction. Preferably from two to four monomer reactants are used, but up to ten monomer reactants can be used. When another monomer reactant M3, present at a stoichiometric quantity p, is used the resulting luminescent polymer has the formula (M1)m(M2)n(M3)p. When yet another monomer reactant M4 is used, the resulting luminescent polymer has the formula (M1)m(M2)n(M3)p(M4)q. The monomers M3, M4, etc. have at least two reactive functional groups and at least one chemically bonded charge transporting chromophore group possessing either electron-withdrawing or electron-donating character. The stoichiometric amounts m, n, p, q, etc. are varied to tune the charge transport property of the resulting conjugated luminescent polymer.
The present invention is also directed to organic electronic devices containing the foregoing conjugated semi-conducting polymers. Such devices typically include at least one thin film of the conjugated polymer coupled to a pair of electrodes. Additional thin films of conjugated luminescent polymer can be used. In such cases, one thin film may be configured to promote electron transport and a second thin film may be tuned to promote hole transport. When the organic luminescent device is fabricated with a plurality of thin films of conjugated luminescent polymer, the thin films are preferably tuned to promote balanced electron and hole transport between the first and second electrodes. Typical organic electronic devices include, but are not limited to, a LED, a thin film transistor, a photovoltaic solar cell, an electrochemical luminescent display device, an electrochromic display device, and an electroluminescent device for active flat-panel display applications.